[saaaaaam]

The battery isn’t for incidental power outages. It’s for when a fire starts somewhere in the building while you are asleep and melts some wiring, knocking out the power to the smoke alarm. An hour later you wake up in a house full of smoke and die.

That’s why smoke alarms have batteries.

[bluGill]

That is also why smoke alarms have interconnects, and there should be smoke detectors all over. The fire should be detected and sound an alarm long before it melts wiring.

Though smoke detectors only detect 40-50% of all fires in time to get out (dual sensor detectors can do much better!). A lot better than nothing, but still not very good.